1. Field of the Invention
The present invention relates to a continuous process for producing a propylene-ethylene block copolymer. More particularly, it relates to a continuous process for producing a propylene-ethylene block copolymer having excellent characteristics of cold shortness and high impact strength which are the same or superior to those of a propylene-ethylene block copolymer obtained by a batch system.
2. Description of the Prior Art
A block co-polymerization of propylene and ethylene has been considered the best method to improve the impact strength under low temperature, which is considered a defect of a crystalline polypropylene.
The block co-polymerization of propylene and ethylene is usually conducted in such a manner that in the first place propylene is polymerized by a plurality of polymerization vessels connected in series and then the co-polymerization of propylene and ethylene is done in the presence of a comparatively large amount of ethylene.
When the polymerization is conducted continuously by using this type of the plurality of vessels connected in series, a certain distribution of the residence time occurs in the polymerization mixture because each vessel reactor is a so-called complete mixing type.
As a result, the effluent discharged from a preceeding vessel will contain various substance such as catalyst grains which are short-passed from the said vessel without giving any contribution to the polymerization, small size polymer grains which had not yet grown sufficiently, polymer grains having high molecular weight and/or larger size which had grown during the long stay in the vessel, etc.
When the effluent is supplied as it is to the vessel for the co-polymerization of propylene and ethylene, the resulting copolymer tends to have inferior impact strength under low temperature than those obtained by a batch process.
Besides, the copolymer will give moldings which are poor in appearance because of a gel formation caused by polymer grains short-passed from propylene polymerization vessel and/or polymer grains grown for a long time in the copolymerization vessel.
Various processes have been proposed to overcome the disadvantages in the continuous polymerization.
The processes using many reactors have been proposed in Japanese Examined Patent Publication No. 25585/1978 and Japanese Unexamined Patent Publication No. 53990/1974. In such processes, a block copolymer having physical properties similar to those of the batch process can be obtained, however, it causes higher cost of construction of the apparatus depending upon increase of the reactors to be uneconomical and it causes complicated quality control of the product.
The process adding an electron donor in the copolymerization of propylene and ethylene in the second step has been proposed in Japanese Examined Patent Publication No. 19542/1969 and Japanese Unexamined Patent Publication No. 115417/1980. Thus, the process has not been effective according to the studies.
The process for reducing a molecular weight of a propylene-ethylene copolymer in the second step has been proposed in Japanese Unexamined Patent Publication No. 61278/1974. The process causes inferior impact strength of the product according to the studies.
In Japanese Unexamined Patent Publication No. 106533/1980, a process is disclosed, in which the slurry in the first stage reaction vessel is discharged from the wall part of said vessel, the discharged slurry being washed co-currently with the same kind of fresh medium as the medium of said slurry to obtain a slurry having substantially less amount of small polymer grains than the original slurry and the slurry being supplied to the second stage reaction vessel.
However, the vessel preferably used in said process tends to cause a vortex, which causes a certain distribution of the grain concentration. Thus, the said process is not of much practical use.
Namely, the formation of a voltex causes not only to decrease the effective volume of the reactor but also to make liquid level control difficult and the distribution of polymer grains in the vessel causes to reduce actual residence time of the grains.
Further, in this process a stable operation cannot always be guaranteed because the polymerization does not proceed uniformly.
Thus, this process is quite difficult to adapt to the actual industrial production field.
There is a process proposed in Japanese Unexamined Publication No. 116716/1980 in which a slurry obtained from the propylene polymerization stage is continuously classified by using a centrifugal separator, a liquid cyclone etc. The slurry obtained by this process contains comparatively large polymer grains, which amount 70-97% of the total polymer obtained, is supplied to the vessel for the copolymerization of ethylene and propylene.
However, the inventors of the present invention have studied and found that the impact strength of the moldings obtained by this process tends to be low.
The inventors of the present invention have done intensive studies to solve the aforesaid problems and reached the present invention by having found that a certain classification system gives a remarkable effect.